There will be a raft of theoretical replied flooding in soon, so on the assumption both are pre-owned, the first big Q is
Do the holes on the transom line up with the ones on the engine?
if so, get yourself a squezy toob of your favourite underwater rated marine sealant, and use Plenty of it round and in the holes. if you coat the length of the fixing bolts with a strip of it & rotate the bolt as you push it trhough slowly you will coat the inside of the hole. Probably best to use M12 Fine thread bolts, unless the clamp has way smaller holes. Nuts on inside /vs outside is a matter of personal choice & aesthetics. Nuts inside may
mean the bottom pair of bolt ends foul your fuel tank if you have a portable sat under the engine.
If not, this is where you have that unnerving pelasure of drilling holes in your boat! Usually best to hang the engine, strap it down (if it doesn't have transom screws like the small Aux type engnes), drill the 4 pilot holes, then remove it again to do the actual drilling, as the clamp will
foul your chuck.
If you have holes that don't line up with the clamp, you'll need to fill them first. Do a search - there's plenty of posts about that game on here.....
Handy tip - if you havean A-frame, fit the steering cable to the engine before you bot it on - otherwise you will discover "Sod's law of a- frames & steering cables"....... Don't ask how I know that
As for rigging, there are many ways of routing cables. If you have nder deck trunking, again have a search, this has been discussed hundreds of times. If you donlt then the tidiest way is to buy ridiculously priced flexible trunking - you'll need sometthing like a 50mm / 2" dia to get absolutely everything through. Option 2 is what I did - take a trip to your local Electrical wholesaler (CEF is a national one) and buy twice as muich 30mm Copex as you need for the run (even buying twice as much it's about 1/10 the price of the "official" 2" stuff per m. One takes the battery cables & one of the throttle / gear cables, the other takes the fuel, remotes wiring & the other throttle or gear cable. Then tie wrap them either side of the steering cable. One good thing about that setup is instead of a 50mm trip hazard across your deck, you end up with a flatter, wider run which is much less of a trip hazard.
Have a look up behind the wheel - some hide the battery in there. If nothng there, a decent battery box & strap it to the floor under the console is likely the best place.
Fuel tanks are another "what fits?" thing - if you donlt ahve a built in tank, I find two Hulk 30L tanks fit nicely side by side under the transom. I have a connector on a 4" length of fuel pipe on each tank and one hose wioth primer. Saves a lot of mess & wansdering loose fuel line back there, as you can cut the hose to be just the right length.
I'm sure there will be more thoughts later, but just post back if you find anything weird.